/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.facebook.presto.operator.scalar;

import com.facebook.presto.metadata.BoundVariables;
import com.facebook.presto.metadata.FunctionRegistry;
import com.facebook.presto.metadata.SqlScalarFunction;
import com.facebook.presto.spi.block.Block;
import com.facebook.presto.spi.block.BlockBuilder;
import com.facebook.presto.spi.block.BlockBuilderStatus;
import com.facebook.presto.spi.type.Type;
import com.facebook.presto.spi.type.TypeManager;
import com.google.common.collect.ImmutableList;

import java.lang.invoke.MethodHandle;

import static com.facebook.presto.metadata.Signature.typeVariable;
import static com.facebook.presto.util.Reflection.methodHandle;
import static com.google.common.base.Preconditions.checkArgument;

public class MapKeys
        extends SqlScalarFunction
{
    public static final MapKeys MAP_KEYS = new MapKeys();
    private static final MethodHandle METHOD_HANDLE = methodHandle(MapKeys.class, "getKeys", Type.class, Block.class);

    public MapKeys()
    {
        super("map_keys", ImmutableList.of(typeVariable("K"), typeVariable("V")), ImmutableList.of(), "array(K)", ImmutableList.of("map(K,V)"));
    }

    @Override
    public boolean isHidden()
    {
        return false;
    }

    @Override
    public boolean isDeterministic()
    {
        return true;
    }

    @Override
    public String getDescription()
    {
        return "Returns the keys of the given map(K,V) as an array";
    }

    @Override
    public ScalarFunctionImplementation specialize(BoundVariables boundVariables, int arity, TypeManager typeManager, FunctionRegistry functionRegistry)
    {
        checkArgument(arity == 1, "map_keys expects only one argument");
        Type keyType = boundVariables.getTypeVariable("K");
        MethodHandle methodHandle = METHOD_HANDLE.bindTo(keyType);
        return new ScalarFunctionImplementation(true, ImmutableList.of(false), methodHandle, isDeterministic());
    }

    public static Block getKeys(Type keyType, Block block)
    {
        BlockBuilder blockBuilder = keyType.createBlockBuilder(new BlockBuilderStatus(), block.getPositionCount() / 2);
        for (int i = 0; i < block.getPositionCount(); i += 2) {
            keyType.appendTo(block, i, blockBuilder);
        }
        return blockBuilder.build();
    }
}
